In the technology of thermal separation, the separation of mixtures of substances of low volatility and high sensitivity to temperatures, frequently poses a difficult problem. The processes of vacuum distillation and molecular distillation were designed to overcome this problem, but these methods fail if azeotropic mixtures are formed, or if the separation factors are very low, the separation factor being the ratio of the distribution coefficients of two substances between the gaseous phase and the liquid phase. Extraction processes are equally unsatisfactory in these cases, particularly if the separation of the extracted substance from the extraction agent--a necessity once the extraction process has been completed--involves thermal burden which is excessively high.
The present invention relates to a process for separating substances of very low volatility, one of which may even be not volatile at all, present in the form of mixtures, from one another by extractive distillation at low temperature. The barely volatile substances are rendered evaporable at low temperatures, with the aid of a gaseous agent under pressures exceeding or equalling its critical pressure, such gaseous agents, because of the supercritical conditions under which they are employed, hereinafter being referred to as "supercritical" agents.
It is well known that supercritical substances under high pressure, have properties which in many respects are similar to those of a liquid. In the state of equilibrium, the concentration of a barely volatile substance in the gaseous phase under pressure, may be higher by many powers of ten than would be expected from its vapor pressure. This effect is partly due to the solubilizing properties of the gaseous phase, and partly to the dissolution of the supercritical gaseous agent in the mixture of barely volatile substances.
In this way it is possible to obtain a distribution of the substances to be separated between a liquid and a gaseous phase, of sufficiently different density.
Suggestions to make use of this effect have been made in the past; cf. U.S. Pat. No. 2,420,185 to Messmore (Philips Petroleum Co.); T. P. Zhuze and A. A. Kapelyushnikov, U.S.S.R. Pat. No. 113,325; and U.S. Pat. No. 3,969,196 to Zosel (Studiengesellschaft Kohle m.b.H.).
On principle, it should be possible to carry out a separation process merely with the aid of the equilibrium between the gaseous and liquid phases. It is usually found, however, that the separation factors between the barely volatile substances to be separated, are not sufficiently modified by the supercritical agent.
Moreover, it is of decisive importance for the economics of a separation process that no compression work should be necessary on the circulating gaseous phase, However, the gas conducted in a cycle must be largely freed from the barely volatile substances before it reenters the principal distillation zone because remixing would impair the quality of the products. Heating of the gaseous phase in order to achieve a sufficient reduction of the content of barely volatile substances, is out of the question, because the temperatures required for this process would frequently result in an excessively high thermal burden on the substances.
The problem of the present invention, therefore, is to provide a process wherein, on the one hand, the distribution ratio of the barely volatile substances to be separated in the gaseous phase and in the liquid phase is improved so that the best possible separation is obtained, and, on the other hand, it is possible to free the gaseous phase conducted in a cycle, from the entrained barely volatile substances, without the necessity of a pressure release or a substantial temperature rise. Manifestly, the problem of the invention has several aspects (improvement of the acceptance of the barely volatile substances by the recycled gaseous phase; freeing the gas from these substances absent any substantial pressure release or temperature increase). In cases in which the pressures are about 1 to 2 times of that the critical pressure of the compressed supercritical gas and the temperatures are a little bit higher (up to about 100.degree. C.) than the critical temperature of the compressed supercritical gas, a rise of temperature the absence of an entrainer may lead to a partial separation of the barely volatile substance dissolved in the compressed gas, as mentioned in U.S. Pat. No. 3,969,196. This separation concerns only part of the dissolved barely volatile substance (about 30 to 60 percent by weight depending on the temperature raise), so that the recycling of the so treated cycle gas into the sump of the first distillation zone leads to so much remixing, that the intended substance separation will be impossible. Partial condensation of the entrainer in a second distillation zone accompanied with the "wash" of the cycle gas in countercurrent according to this invention leads to improved separation of the barely volatile components dissolved in the cycle gas residue is less than 0.2 percent by weight of the originally dissolved components), so that the recycling of the cycle gas into the sump of the first distillation zone will not lead to a disadvantageous remixture. While the former aspect has been recognized by the above-noted U.S. Pat. No. 3,969,196 which has attempted to solve that part of the problem in a manner distinct from the present approach, the latter aspect has not been dealt with in the prior patent which proposes precisely that pressure release or temperature rise whose elimination is one of the primary objectives of the present invention.